The invention disclosed herein relates to lithographic printing plates. Plates of interest commonly have a solvent-soluble, radiation-polymerizable, oleophilic resin coating on a hydrophilic substrate. In conventional practice, after image-wise exposure at ultraviolet (UV), visible (violet), or infrared (IR) wavelengths, the plates are developed with solvent to remove the unexposed areas of the coating by dissolution, thereby producing a substantially planographic pattern of oleophilic and hydrophilic areas. The developed plates are then ready for mounting on a cylinder of a printing press, where the plates are subjected to fountain fluid and ink for transfer of ink to a target surface according to the pattern of oleophilic and hydrophilic areas on the plate. Lithography is operates on the fundamental principle that oil and water are immiscible.
In modern offset printing, a photosensitive coating is typically applied to an anodized aluminum sheet to produce a printing plate. An imagewise exposure of the photosensitive coating to radiation initiates photopolymerization in the image areas, and consequently an image on the surface of the plate. On a printing press, ink is attracted to the imaged portions of the coating and the water (i.e., aqueous fountain solution) is attracted to the non-imaged plate surface, which is often anodized aluminum. The ink is subsequently transferred from the imaged areas of the plate to a rubber blanket sleeve, and then to the print medium to produce the printed product.
Digitization of the imaging process has increased in recent years, resulting in simplification and improvement in image quality. Images are typically transmitted digitally from computer to the plate surface by means of laser direct imaging. Laser direct imaging devices have been commercialized with ultra-violet, visible, and infrared laser. Of these, infrared (830 nm) and violet (405 nm) have received the most commercial acceptance for the imaging of printing plates.
Infrared is often considered to offer the highest quality image reproduction, and also allows plates to be prepared in a white light working environment. However, infrared imaging devices are costly to purchase and maintain, typically requiring replacement of high powered laser heads after as few as 3000 working hours.
Violet imaging provides high quality reproduction in a bright yellow light working environment. Compared to infrared equipment, violet imaging equipment is relatively inexpensive to purchase and maintain and provides a high level of productivity. A violet laser may last up to as many as 10,000 working hours, and replacement costs are relatively low. Violet imaging equipment has improved to the point where resolutions in excess of 200 lines per inch are possible, and imaging limitations are more attributable to the photosensitive plates rather than the exposure equipment.
Thermally imageable plates are commercially available, which require no pre-heat step prior to development. These plates usually have relatively low resolution and short press lives. The main reason for this is that they need more imaging exposure energy in order to gain integrity for the image. When an image is created in this manner it causes the “dots” or pixels that form the image to gain surface area. This phenomenon is called “dot gain” and causes degradation in the resolution of the plate.
As an alternative, the plate can be exposed at lower imaging energy and then pre-heated before development, but “dot gain” still often occurs. However, in this case it is the excess energy of the heater that causes the “dot gain”. This energy (in the form of heat) forces the polymerization to continue not only in the center of the dots (which is needed for longer press life) but it also causes the dots to grow out from the edges.
The energy available for plates exposed with infrared laser radiation is in the order of 50 to 200 mJ/cm2, approximately 1000 times greater than a violet laser output. It is possible to produce photopolymerizable compositions for infrared imaging, which do not require a “pre-heat” step, a heating process used to enhance the image prior to development processing. For example, photoinititor compositions capable of producing such an image without the pre-heat exist which substantially simplify plate processing. Plates capable of development in simple one-bath processors through environmentally friendly developers are available, as are plates that develop directly on press in the ink and fount train.
There are known advantages to imaging coatings sensitive to violet and ultra-violet (UV) energy. However, a common disadvantage to violet imagine arises because typical imaging equipment cannot generate high intensity beams, so preheating after imaging is often required, thereby adding complexity to the production process.
Plates imaged by visible violet radiation are exposed with energies ranging from 50 to 200 μJ/cm2. The amount of energy is relatively low and considered insufficient to fully polymerize the image. A post-exposure pre-heating step is typically required to harden the image and improve durability and adhesion to the substrate to commercially acceptable levels. Processing with a pre-heat step necessarily adds complexity to the production process by requiring processors equipped with ovens, which increases overall cost of production and maintenance often to the level of offsetting the advantages of using violet imaging equipment over infrared, or other common techniques. Variability of temperature can impact image resolution and durability, thereby the quality of the printed product. Furthermore, due to the pre-heating requirement, known violet plates cannot be developed through simple one-bath processors or directly on press. For these reasons, plates exposed by violet laser are often utilized in lower quality printing environments such as newsprint.
Attempts have been made to produce violet plates which do not require preheat, including processes of producing an image by writing with a violet laser, then maintaining the plate at ambient conditions for at least 2.5 minutes to enhance polymerization and harden the image prior to processing. Such an approach is inconvenient for existing, fast paced, automated production environments and has been known to yield an image with sufficient on-press durability.
Regardless of how plate manufacturers and end users make the tradeoff between infrared and violet imaging techniques and plates, in conventional solvent based development of negative, actinically imageable lithographic plates, no substantial further cross-linking can be achieved in the image areas after development of the plate in solvent. Any coating material in the image areas that did not react with the radiation, is dissolved and therefore removed from the image areas during the development step.
There remains a need for photopolymer printing plates with improved durability on press, especially when the pre-heat step is eliminated during the development process. In particular, there remains a need for photopolymer printing plates exposed by visible violet laser with improved durability on press, especially when plates are not subject to a pre-heat step.